Print this pageThe 
Relationship
between Bed Curvature and Fracture
Occurrence in a Fault-Propagation Fold
HENNINGS, PETER H., and JON E. OLSON
The geometry and intensity of fractures in the Cretaceous Frontier Sandstone
of the hydrocarbon-productive Oil Mountain Anticline (OMA) are, in part, related
to the deformation path of the parent structure. OMA is the southern culmination
of a line of anticlines produced by southwest-dipping backthrusts emanating from
the leading edge of the Casper Arch thrust. OMA results from folding in advance
of a thrust that steps up-section from a layer-parallel detachment at the base
of the Ordovician Bighorn Fm, to its tip within the Chugwater Fm. Additional
layer-parallel shear, distributed throughout the detached portion of the
anticline, produced overturning of the forelimb.
A NW striking set of regional joints can be observed in nearby flat-lying beds of Frontier Fm. Younger systematic joint sets are observed only in folded beds and roughly parallel the direction of Laramide shortening. Joints occur in domains of varying intensity. Some joint swarms developed into tear faults, with the older joints accommodating shear and the newer joints forming in a rotated stress field.
We correlate the geometry and evolution of joints to a 3D kinematic model of
bed curvature generated by volumetric restoration. Where the gently-dipping
limb
of the fold merges into the plunging nose, the sandstone beds accommodated the
curvature by an evolution of joints into small faults. The faults separate large
blocks which remained intact. At the nose of the fold where curvature is
highest, the Frontier Formation. deformed by pervasive shearing. Along the steep
limb, jointing and minor faulting are again dominant.